How have hybrid zones contributed to our understanding of reproductive isolation?

Just some background information, hybrid zones are regions in which members of different species meet and mate, producing at least some offspring of mixed ancestry.How have hybrid zones contributed to our understanding of reproductive isolation?
Hybrid zones are associated with incomplete reproductive isolation. The hybrid offspring have intermediate characteristics, and hybrid zones themselves provide natural laboratories for the study of speciation. E.g. European toads of the genus Bombina. New species may arise through hybridization (known as the Hybridization Hypothesis).